1. Field of the Invention
The invention relates to membrane gas separations. More particularly, it relates to the use of membranes for the production of oxygen, or oxygen enriched air, from air.
2. Description of the Prior Art
Oxygen gas is a valuable commodity product that is widely used throughout industry. The conventional means for producing oxygen is by the cryogenic distillation of air. For large-scale production, cyrogenic distillation is very efficient and produces a high purity oxygen product. For smaller volume applications, particularly where the costs of distributing high purity oxygen gas or liquid to a point of use are high, however, various "on-site" oxygen production systems may be highly desirable and preferable to cryogenic distillation. In recent years, pressure swing adsorption (PSA) processes and systems have been developed to serve such applications. Thus, on-site PSA systems can be developed for the production of relatively large volumes of oxygen product, and desirably, can be scaled down to a small size for lower volume applications. For example, very small PSA systems are used to generate oxygen for hospital and home-care applications, as in the generation and supply of oxygen for individuals suffering from emphysema and the like.
There are a variety of other applications, particularly for the enhancement of combustion processes, where low purity oxygen, or even oxygen enriched air, is satisfactory, or even preferred, on an overall evaluation basis. For such identified applications, such low purity oxygen or oxygen enriched air is usually provided by diluting high purity oxygen produced in cryogenic distillation systems with ambient air.
As a result of such experiences, the art has come to appreciate the significant number of actual and potential uses for oxygen gas having varying degrees of purity level and available in widely differing volume apparatus. In light of such appreciation, the art has recently developed a considerable interest in the use of membrane permeation technology for the production of oxygen and nitrogen from air. Membrane permeation processes are attractive because of their inherent simplicity, including the absence of any requirement for moving mechanical equipment other than that needed to compress feed air for passage to the membrane system. In order to achieve Practical oxygen permeation rates for commercial applications, the permeable membrane must be very thin, possess a large surface area, and be free of pinholes and other defects that would negate the selectivity of gas separation obtainable by the membrane. These requirements have been satisfied to a considerable extent by the development of asymmetric and composite-type hollow fiber membranes. By proper disposition and assembly of hollow fibers into membrane bundles, it is possible to produce membrane modules suitable for the passage of high pressure feed air along either the shell side or the bore side of the fibers. The feed air becomes progressively leaner in its more highly permeable components as it passes on one side of the membrane module, and progressively richer in the less permeable components, ultimately being withdrawn from the membrane module as the non-permeate or "retentate" stream. The permeate gas, i.e. the gas that has passed through the thin separation region of the membrane, flows along the opposite side, either bore side or shell side, of the fibers and is separately withdrawn from the membrane module.
Various mathematical models have been developed to describe the operating characteristics of membrane permeation modules, as for example as shown by C. Y. Pan and H. W. Habgood, in Can. J. Chem., Eng. 56 (1978) pp. 197-205. Most hollow fiber membrane modules are found to operate according to the "crossflow" model, wherein the composition of the local permeate, on the low side of the skin or separation layer portion of the membrane, is considered not to mix with the bulk permeate gas stream. According to this model, the direction of the permeate flow is inconsequential, and the permeate stream can be withdrawn from either end of the module. Since no beneficial effect is seen in employing a permeate purge stream under such circumstances, most permeators are designed as three-port devices with no provision for the addition of purge gas thereto. The three ports employed correspond to the feed gas input, the retentate output, and the permeate output.
If a composite hollow fiber membrane is formed with a thin separation layer of membrane material coated over a relatively high porosity substrate, it is possible to produce a permeator that exhibits a high degree of radial mixing on both the permeate and non-permeate sides of the membrane. By winding such composite hollow fibers in an ordered helical manner, such that all fibers are of the same length, the membrane module can be made to perform more favorably than would be predicted by the crossflow model. In such cases, the modules tend to follow the desirable "cocurrent" or the "countercurrent" permeation models, depending on the direction of permeate gas flow relative to the flow of retentate gas. In most cases, the countercurrent flow pattern yields the best results for practical commercial operations. Desirable countercurrent modules can be constructed with four ports, so that a low pressure purge gas stream can be introduced to said modules on the permeate side thereof.
In general, organic polymers suitable for use in the formation of the separation layer of composite membranes are more permeable to oxygen than to nitrogen. Thus, when so used in a membrane module for air separation purposes, such separation layer materials will cause the permeate to become enriched in oxygen, as the more readily permeable component of feed air, while the retentate becomes enriched in nitrogen. The degree of air separation achieved is related to the ratio of the permeability coefficients of oxygen and nitrogen for the separation layer material, i.e. the separation factor. Typical separation factors of commercially feasible polymeric materials for air separation range generally from 3 to 10. As is known in the art, most polymers having high selectively, i.e. high separation factors, also tend to have relatively low production or permeability characteristics.
While membrane permeation processes have been considered for air separation applications for either oxygen or nitrogen production, it is much easier to devise an economically attractive process for producing nitrogen than for oxygen production. As air is passed at high pressure along a membrane surface, more oxygen tends to permeate through the membrane material than nitrogen. Thus, the feed air stream becomes leaner in oxygen, and richer in nitrogen, as it passes along the membrane from the feed input port to the retentate output port. By increasing the length of the flow passage, or by reducing the feed air flow rate, the degree of nitrogen product purification achieved can be increased. In this manner, most of the oxygen can be removed in a single step process to produce a relatively pure nitrogen product. The permeate is thereby enriched in oxygen, but, since a portion of the nitrogen in the feed air also permeates through the membrane, the purity of the permeate gas is necessarily limited. Furthermore, for a given flow rate, the longer the membrane fiber is made, the more nitrogen is permeated along with oxygen, and the lower the oxygen purity of the permeate gas obtained. The highest oxygen purity is achieved when the so-called stage cut is very small, so that nearly pure air exists along the retentate side of the membrane. Operations at very low stage cuts, however, are very inefficient. As a result, practical single-stage membrane processes for oxygen production are severely limited with respect to the oxygen purity levels that can be produced.
Permeable membrane air separation processes have been devised for producing oxygen at higher purity levels by the use of multiple membrane stages. One such membrane process for increasing oxygen purity is by the use of a cascade process. In this approach, the permeate from an initial membrane stage, which has been enriched in oxygen, is recompressed and passed through a second membrane stage, whereby the permeate is further enriched in oxygen. The permeate from the second stage can, if desired, be passed to additional stages, ultimately leading to the recovery of a high purity oxygen product. Such cascade processes, however, require that the permeate streams be recompressed before use in subsequent stages. This requires the use of multiple compressors, which must be synchronously controlled.
Another approach for increasing oxygen purity is by employing two membrane permeators in series, and recycling the permeate gas of the second stage to the feed end of the first membrane stage. The oxygen concentration of the product gas recovered as permeate from the first stage can be controlled by adjusting the overall stage cut of the system. This approach is capable of producing relatively high purity oxygen provided that the recycle rate of permeate from the second stage is sufficiently high. At a high recycle rate, the recycle stream is rich in oxygen compared to air. This recycle stream is blended with input feed air, increasing its oxygen concentration and thus raising the oxygen concentration of the permeate recovered from the first stage membrane permeator.
In this recycle process using two membranes in series, the high pressure retentate from the first stage serves as feed gas for the second stage. The permeate from the second stage can be oxygen rich, depending on the stage cut, and this stream is blended with the feed air to form the feed input to the first stage. The oxygen concentration of the recycled stream is greater than 21%, i.e. air. The first stage permeate is recovered as product gas.
The operating characteristics of this recycle process depend on the separation factor of the membrane, the feed/permeate pressure ratio employed, the relative membrane surface areas of the two stages, and the overall stage cut. It has been found that the maximum oxygen purity obtainable increases substantially as more of the membrane area is allocated to the second stage, at fixed separation factor and pressure ratio conditions. The oxygen concentration varies with the stage cut, and it peaks at stage cuts in the range of 0-21% corresponding to the oxygen concentration in air. In the vicinity of the peak, the stage cut can be varied appreciably without greatly altering the product oxygen concentration. Thus, there is a wide range of possible operating conditions for this process. In fact, if more than about 70% of the surface area is allocated to the second stage, the process can yield product oxygen concentrations that are above the single stage theoretical limit, e.g. about 50% oxygen by volume at a separation factor of 6.
In spite of the advantages of the two stage recycle process, it suffers in efficiency, primarily because of the blending of gas streams of different composition at the feed point. This blending generates entropy that degrades the overall process efficiency. Thus, there remains in the art a need for an improved, more efficient membrane oxygen process and system.
It is an object of the invention to provide an improved membrane gas separation process and system.
It is another object of the invention to provide an improved membrane process and system for the production of oxygen, or oxygen enriched air, from air.
It is a further object of the invention to provide a more efficient membrane oxygen production process and system.
With these and further objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.